The overall objective of this research is to develop statistical methods for estimating vaccine efficacy and effectiveness in the field, and to characterize complex and long-term properties of vaccines in individuals and populations. The specific aims are (1) to model unmeasured heterogeneity in susceptibility in vaccinated and unvaccinated populations in order to obtain interpretable parameters of protective vaccine efficacy, (2) to develop methods to estimate complex characteristics of vaccines such as sensitivity to boosting of the immune response by natural re-infection and waning of immunity, and (3) to evaluate the efficacy and effectiveness of vaccines that modify the infectiousness of an infected vaccinated person. The statistical methods we will use include frailty survival models, stochastic random effects survival models, epidemic models, linear random effects models, and Bayesian methods. Opportunities for reanalysis of data include cholera vaccine trials in Bangladesh, influenza vaccine studies in Michigan, pre-licensure chickenpox vaccine studies, measles outbreak studies, Hemophilus influenzae b vaccine studies in Georgia, and longitudinal studies of antibody titers to hepatitis B and chickenpox vaccines. Collaborative design of studies include the new cholera vaccine studies in Bangladesh and measles outbreaks in highly vaccinated populations.